Wind turbine blades are typically made from two blade shell halves of fibre-reinforced polymer. The two blade shell halves are glued together along edges of the shell halves and additionally one or more spars or webs extending in the longitudinal direction are glued to each of the shell halves to provide stiffness to the blade. It is, however, also possible to mould the blades by so-called hollow moulding, whereby the blades are made of a single shell structure only. The blades or blade halves are typically made by vacuum infusion, especially vacuum-assisted resin transfer moulding (VARTM) or by using so-called prepregs, i.e. by using fibre material pre-impregnated with resin. The fibre material is typically in the form of rovings, i.e. fibre bundles, bands of rovings or mats which may be felt mats of single fibres or woven mats of fibre rovings, whereby the mats may be unidirectional mats or multi-directional mats. The resin or polymer used is typically polyester, epoxy or vinylester. The reinforcement fibre material is typically fibreglass. However, carbon fibres are also used, as they are stiffer and have a higher E-modulus than glass fibres, and thereby provide for a higher degree of stiffness and/or a lower weight of the wind turbine blade. Additionally, it is known to use metal fibres, i.e. metal filaments in combination with glass fibres or carbon fibres.
Other types of reinforcement fibres are aramid fibres and other types of polymer fibres, natural fibres, such as hemp fibres, flax fibres and bamboo fibres may also be used for the manufacture of wind turbine blades.
WO 2006/082479 discloses a wind turbine blade and a method for preparing a wind turbine blade shell member comprising a plurality of elements of cured fibre-reinforced sheet material, such as elements of carbon fibres, glass fibres and/or wood fibres. WO 2010/006807 discloses wind turbine blade formed of a fibrous composite material including two or more different types of carbon fibres having a different E-modulus to each other and wherein the proportions of the different types of carbon fibres vary in the longitudinal direction of the blade.
WO 2003/078832 discloses a wind turbine blade of fibre-reinforced polymer including a first type of fibres, such as glass fibres, of a first stiffness and a second type of fibres, such as carbon fibres, of a different stiffness. In a transition region between the two types of fibres the quantitative ratio of the two types of fibres varies continuously in the longitudinal direction of the blade.
Unpublished European patent application No. 11161889.8 discloses a wind turbine blade as stated in the preamble to claim 1, wherein the transition region has a gradually changing proportion between the first and the second reinforcement fibre material. The first region extends in the root region of the blade and the first reinforcement fibre material is a metal.
An abrupt transition between types of fibres having differing stiffness, i.e. E-modulus, causes heavy stress concentration. Especially transitions between glass fibres and carbon fibres may cause problems, as glass fibres have an E-modulus of about 70 GPa, while the E-modulus of carbon fibres is about 230 GPa and 600 GPa or more. By providing a gradual transition between the two types of fibres, the stress concentration may be mitigated, reduced or eliminated.
In order to compensate for stress concentrations when using reinforcement fibres with differing E-moduli in composites, it is possible to provide a local thickening in the transition area between the two different fibres and thereby limit the risk of failure due to stress concentrations. One drawback of such a solution is, however, increased weight due to the increased use of fibres, e.g. glass fibres, in the transition area between glass fibres and carbon fibres.